Vascular tissue engineering is becoming an important area of research in recent years due to pathological diseases (e.g. diabetic blindness, gangrene) resulting from deficient angiogenesis. Angiogenesis is a natural biological process that involves the formation of new blood capillaries from pre-existing ones (Nakatsu et al., 2003), providing adequate blood supply for tissues to obtain nutrients and oxygen for survival. In healthy individuals, angiogenesis is regulated through the production of a balanced amount of growth and inhibitory factors. The loss of this balance results in formation of excessive or insufficient blood capillaries, both of which are directly related to pathological diseases. Excessive formation of blood capillaries support tumor metastasis to other regions of the body (Stephanou et al., 2006), whereas a lack of new blood capillary formation hinders the growth and development of tissues, eventually leading to pathology (e.g. in cases of heart failure, diabetic blindness, and gangrene).
Organ transplants and blood vessel prosthesis are ways in which such clinical conditions caused by a lack of angiogenesis can be treated. However, these may not be ideal treatment options due to the diminishing availability of organs and the high risk of infection that prosthetic blood vessels may bring. Autografts have also been clinically used for the replacement of damaged blood vessels. However, such a procedure is not popular because it requires multiple operations.
Therefore, the current approach to treating such diseases involves inducing the formation of blood vessels by re-initiating the angiogenic process. Current methods used include in vivo gene therapy via the injection of plasmids containing VEGF and bFGF cDNA into ischemic tissues (Bauters et al., 1994), and targeted in vivo injection of growth factor proteins in its soluble form (Baffour et al., 1992). These approaches may be less effective than desired due to diffusion of growth factors away from the ischemic site following injection (Phelps et al., 2009).
Current efforts in tissue engineering focus on development of hydrogels to support the formation of neo-vessels. Hydrogels are synthetic three-dimensional, biodegradable scaffolds that may be used to encapsulate growth factors to induce proliferation and migration of endothelial cells from quiescent vessels into the matrix. Alternatively, cells can be encapsulated within a hydrogel and prevascularization is stimulated before implantation of hydrogel into the host.
A variety of hydrogels have been proposed for vascular tissue engineering, some of which include fibrin gel, collagen gels, and gels formed using synthetic polymers (e.g. polyethylene glycol, dextran) (Sieminski et al, 2004; Moon et al., 2009; Phelps et al., 2009).
Most research focuses on the use of fibrin gel as a matrix for stimulating blood vessel formation (Collen et al., 1998; Nakatsu et al., 2003; Urech et al, 2004). Fibrin, a naturally occurring clotting material involved in the process of blood coagulation is a popular biodegradable scaffold used in vascular tissue engineering due to its ability to support cell adhesion and proliferation, and most importantly, angiogenesis. Furthermore, autologous fibrinogen, the precursor of fibrin, can be isolated from the patient's own blood, thus avoiding the risks of immune rejections (Aper et al., 2006; Jockenhoevel et al., 2001). In addition, fibrinogen contains integrin αvβ3 (Perumal et al., 1996) and cell adhesion molecules like L1 and ephrin B2 that allow the adhesion of endothelial cells, thereby, facilitating their spreading, traction, and proliferation on the fibrin fibers.
However, fibrin is degraded rapidly by proteases such as plasmin and matrix metalloproteinases (MMPs), undermining its potential as a tissue engineering scaffold, as it is desirable for the fibrin gel shape to remain intact until mature vessels can be formed. Previously, protease inhibitors have been used to overcome the rapid rate of degradation. The use of aprotinin, a protease inhibitor derived from bovine, however, has been suspended recently due to associations with serious adverse effects. Tranexamic acid, an alternative fibrinolysis inhibitor, is also associated with side effects (Furst et al., 2006). Increasing the concentration of fibrinogen in order to improve degradation resistance leads to an increased mechanical strength, and may result in a gel with too great a stiffness that is not suitable for supporting capillary formation.
Several studies have also reported the successful formation of angiogenic sprouts observed in fibrin gels (Collen et al., 1998; Urech et al., 2004). In contrast, in vitro angiogenesis models using collagen gels or matrigel failed to model the formation of sprouts. Any lumens that were observed were thin and slit-like in appearance (Nakatsu et al., 2003). Furthermore, human umbilical vein endothelial cell (HUVEC), widely known as the canonical cell line for in vitro angiogenesis assays, responded most efficiently to interactions with fibrin (Ingber and Folkman, 1989).
Thus, there exists a need for an alternative tissue scaffold that can be used to support angiogenesis.